1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus having the function of monitoring the flow of a contrast medium into a region of interest by a preparation scan.
2. Description of the Related Art
In contrast medium imaging, it is important to perform a scan (main scan) at a proper timing when the contrast medium injected into a subject to be examined flows into a target slice. For this reason, a scan (preparation scan) at a dose lower than that for the main scan and image reconstruction with a low resolution are repeated before the main scan. When an imaging operator visually checks the density of contrast medium at the region of interest of a tomographic image and recognizes that the density has reached a certain level, the operator inputs a trigger for the main scan. Upon reception of the trigger, the apparatus stops the preparation scan and starts the main scan.
Recently, a technique called a real preparation scan, which automatically sets a main scan start timing, has been widely used. According to this technique, as shown in FIG. 1A, the CT value of a region of interest is extracted from the tomographic image data obtained by a preparation scan, and the extracted value or an average CTave. is compared with a threshold TH. At time t when the CT value or average CTave. has reached the threshold TH, a shift from the preparation scan to the main scan is started.
In a diagnosis, a doctor sees the consecutive tomographic images acquired by the main scan as a moving image, and in many cases, objectively checks variations in the contrast medium density of the region of interest over time with a time density curve. This operation is a so-called dynamic study.
The precision of this dynamic study is susceptible to the movement of the tissue due to breathing and pulsation as well as the body movement of a subject to be examined. The precision is affected more as the size of the region of interest decreases.
For this reason, before the main scan, the examiner instructs the subject to hold his/her breath and suppress body movement by using a loudspeaker or the like. An apparatus for automatically outputting such instructions has also been developed. In this apparatus, voice data about breath holding, suppression of body movement, and the like are held. As shown in FIG. 1B, at time t when the CT value or average CTave. has reached the threshold TH, the voice data is played back (voice guidance).
In this case, however, the main scan starts with a delay corresponding to the time required to play back the voice data from time t when the CT value or average CTave. has reached the threshold TH.